Auto merge of #119899 - onur-ozkan:redesign-stage0-std, r=albertlarsan68,jieyouxu,mark-simulacrum,kobzol,jyn514,Noratrieb,WaffleLapkin,RalfJung,bjorn3

redesign stage 0 std

### Summary

**Blog post: https://blog.rust-lang.org/inside-rust/2025/05/29/redesigning-the-initial-bootstrap-sequence/**

This PR changes how bootstrap builds the stage 1 compiler by switching to precompiled stage 0 standard library instead of building the in-tree one. The goal was to update bootstrap to use the beta standard library at stage 0 rather than compiling it from source (see the motivation at https://github.com/rust-lang/compiler-team/issues/619).

Previously, to build a stage 1 compiler bootstrap followed this path:

```
download stage0 compiler -> build in-tree std -> compile stage1 compiler with in-tree std
```

With this PR, the new path is:

```
download stage0 compiler -> compile stage1 compiler with precompiled stage0 std
```

This also means that `cfg(bootstrap)`/`cfg(not(bootstrap))` is no longer needed for library development.

### Building "library"

Since stage0 `std` is no longer in-tree `x build/test/check library --stage 0` is now no-op. The minimum supported stage to build `std` is now 1. For the same reason, default stage values in the library profile is no longer 0.

Because building the in-tree library now requires a stage1 compiler, I highly recommend library developers to enable `download-rustc` to speed up compilation time.

<hr>

**Blog post: https://blog.rust-lang.org/inside-rust/2025/05/29/redesigning-the-initial-bootstrap-sequence/**

If you encounter a bug or unexpected results please open a topic in the [#t-infra/bootstrap](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap) Zulip channel or create a [bootstrap issue](https://github.com/rust-lang/rust/issues/new?template=bootstrap.md).

(Review thread: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Review.20thread.3A.20stage.200.20redesign.20PR/with/508271433)

~~Blocked on https://github.com/rust-lang/rust/pull/122709~~

try-job: dist-x86_64-linux
try-job: `x86_64-msvc*`
try-job: `x86_64-apple-*`
try-job: `aarch64-apple`
try-job: x86_64-gnu
try-job: `x86_64-gnu-llvm*`

5 files changed, 53 insertions, 111 deletions

diff --git a/src/doc/rustc-dev-guide/src/building/bootstrapping/what-bootstrapping-does.md b/src/doc/rustc-dev-guide/src/building/bootstrapping/what-bootstrapping-does.md
index a2930b3e427..2793ad43815 100644
--- a/src/doc/rustc-dev-guide/src/building/bootstrapping/what-bootstrapping-does.md
+++ b/src/doc/rustc-dev-guide/src/building/bootstrapping/what-bootstrapping-does.md
@@ -45,13 +45,13 @@ compiler.
 
 ```mermaid
 graph TD
-    s0c["stage0 compiler (1.63)"]:::downloaded -->|A| s0l("stage0 std (1.64)"):::with-s0c;
+    s0c["stage0 compiler (1.86.0-beta.1)"]:::downloaded -->|A| s0l("stage0 std (1.86.0-beta.1)"):::downloaded;
     s0c & s0l --- stepb[ ]:::empty;
-    stepb -->|B| s0ca["stage0 compiler artifacts (1.64)"]:::with-s0c;
-    s0ca -->|copy| s1c["stage1 compiler (1.64)"]:::with-s0c;
-    s1c -->|C| s1l("stage1 std (1.64)"):::with-s1c;
+    stepb -->|B| s0ca["stage0 compiler artifacts (1.87.0-dev)"]:::with-s0c;
+    s0ca -->|copy| s1c["stage1 compiler (1.87.0-dev)"]:::with-s0c;
+    s1c -->|C| s1l("stage1 std (1.87.0-dev)"):::with-s1c;
     s1c & s1l --- stepd[ ]:::empty;
-    stepd -->|D| s1ca["stage1 compiler artifacts (1.64)"]:::with-s1c;
+    stepd -->|D| s1ca["stage1 compiler artifacts (1.87.0-dev)"]:::with-s1c;
     s1ca -->|copy| s2c["stage2 compiler"]:::with-s1c;
 
     classDef empty width:0px,height:0px;
@@ -62,19 +62,21 @@ graph TD
 
 ### Stage 0: the pre-compiled compiler
 
-The stage0 compiler is usually the current _beta_ `rustc` compiler and its
+The stage0 compiler is by default the very recent _beta_ `rustc` compiler and its
 associated dynamic libraries, which `./x.py` will download for you. (You can
-also configure `./x.py` to use something else.)
+also configure `./x.py` to change stage0 to something else.)
 
-The stage0 compiler is then used only to compile [`src/bootstrap`],
-[`library/std`], and [`compiler/rustc`]. When assembling the libraries and
-binaries that will become the stage1 `rustc` compiler, the freshly compiled
-`std` and `rustc` are used. There are two concepts at play here: a compiler
-(with its set of dependencies) and its 'target' or 'object' libraries (`std` and
-`rustc`). Both are staged, but in a staggered manner.
+The precompiled stage0 compiler is then used only to compile [`src/bootstrap`] and [`compiler/rustc`]
+with precompiled stage0 std.
+
+Note that to build the stage1 compiler we use the precompiled stage0 compiler and std.
+Therefore, to use a compiler with a std that is freshly built from the tree, you need to
+build the stage2 compiler.
+
+There are two concepts at play here: a compiler (with its set of dependencies) and its
+'target' or 'object' libraries (`std` and `rustc`). Both are staged, but in a staggered manner.
 
 [`compiler/rustc`]: https://github.com/rust-lang/rust/tree/master/compiler/rustc
-[`library/std`]: https://github.com/rust-lang/rust/tree/master/library/std
 [`src/bootstrap`]: https://github.com/rust-lang/rust/tree/master/src/bootstrap
 
 ### Stage 1: from current code, by an earlier compiler
@@ -84,16 +86,14 @@ The rustc source code is then compiled with the `stage0` compiler to produce the
 
 ### Stage 2: the truly current compiler
 
-We then rebuild our `stage1` compiler with itself to produce the `stage2`
+We then rebuild the compiler using `stage1` compiler with in-tree std to produce the `stage2`
 compiler.
 
-In theory, the `stage1` compiler is functionally identical to the `stage2`
-compiler, but in practice there are subtle differences. In particular, the
-`stage1` compiler itself was built by `stage0` and hence not by the source in
-your working directory. This means that the ABI generated by the `stage0`
-compiler may not match the ABI that would have been made by the `stage1`
-compiler, which can cause problems for dynamic libraries, tests, and tools using
-`rustc_private`.
+The `stage1` compiler itself was built by precompiled `stage0` compiler and std
+and hence not by the source in your working directory. This means that the ABI
+generated by the `stage0` compiler may not match the ABI that would have been made
+by the `stage1` compiler, which can cause problems for dynamic libraries, tests
+and tools using `rustc_private`.
 
 Note that the `proc_macro` crate avoids this issue with a `C` FFI layer called
 `proc_macro::bridge`, allowing it to be used with `stage1`.
@@ -101,9 +101,10 @@ Note that the `proc_macro` crate avoids this issue with a `C` FFI layer called
 The `stage2` compiler is the one distributed with `rustup` and all other install
 methods. However, it takes a very long time to build because one must first
 build the new compiler with an older compiler and then use that to build the new
-compiler with itself. For development, you usually only want the `stage1`
-compiler, which you can build with `./x build library`. See [Building the
-compiler](../how-to-build-and-run.html#building-the-compiler).
+compiler with itself.
+
+For development, you usually only want to use `--stage 1` flag to build things.
+See [Building the compiler](../how-to-build-and-run.html#building-the-compiler).
 
 ### Stage 3: the same-result test
 
@@ -114,10 +115,11 @@ something has broken.
 ### Building the stages
 
 The script [`./x`] tries to be helpful and pick the stage you most likely meant
-for each subcommand. These defaults are as follows:
+for each subcommand. Here are some `x` commands with their default stages:
 
-- `check`: `--stage 0`
-- `doc`: `--stage 0`
+- `check`: `--stage 1`
+- `clippy`: `--stage 1`
+- `doc`: `--stage 1`
 - `build`: `--stage 1`
 - `test`: `--stage 1`
 - `dist`: `--stage 2`
@@ -191,8 +193,8 @@ include, but are not limited to:
   without building `rustc` from source ('build with `stage0`, then test the
   artifacts'). If you're working on the standard library, this is normally the
   test command you want.
-- `./x build --stage 0` means to build with the beta `rustc`.
-- `./x doc --stage 0` means to document using the beta `rustdoc`.
+- `./x build --stage 0` means to build with the stage0 `rustc`.
+- `./x doc --stage 0` means to document using the stage0 `rustdoc`.
 
 #### Examples of what *not* to do
 
@@ -208,9 +210,6 @@ include, but are not limited to:
 
 ### Building vs. running
 
-Note that `build --stage N compiler/rustc` **does not** build the stage N
-compiler: instead it builds the stage N+1 compiler _using_ the stage N compiler.
-
 In short, _stage 0 uses the `stage0` compiler to create `stage0` artifacts which
 will later be uplifted to be the stage1 compiler_.
 
@@ -268,23 +267,6 @@ However, when cross-compiling, `stage1` `std` will only run on the host. So the
 
 (See in the table how `stage2` only builds non-host `std` targets).
 
-### Why does only libstd use `cfg(bootstrap)`?
-
-For docs on `cfg(bootstrap)` itself, see [Complications of
-Bootstrapping](#complications-of-bootstrapping).
-
-The `rustc` generated by the `stage0` compiler is linked to the freshly-built
-`std`, which means that for the most part only `std` needs to be `cfg`-gated, so
-that `rustc` can use features added to `std` immediately after their addition,
-without need for them to get into the downloaded `beta` compiler.
-
-Note this is different from any other Rust program: `stage1` `rustc` is built by
-the _beta_ compiler, but using the _master_ version of `libstd`!
-
-The only time `rustc` uses `cfg(bootstrap)` is when it adds internal lints that
-use diagnostic items, or when it uses unstable library features that were
-recently changed.
-
 ### What is a 'sysroot'?
 
 When you build a project with `cargo`, the build artifacts for dependencies are
@@ -459,7 +441,6 @@ compiler itself uses to run. These aren't actually used by artifacts the new
 compiler generates. This step also copies the `rustc` and `rustdoc` binaries we
 generated into `build/$HOST/stage/bin`.
 
-The `stage1/bin/rustc` is a fully functional compiler, but it doesn't yet have
-any libraries to link built binaries or libraries to. The next 3 steps will
-provide those libraries for it; they are mostly equivalent to constructing the
-`stage1/bin` compiler so we don't go through them individually here.
+The `stage1/bin/rustc` is a fully functional compiler built with stage0 (precompiled) compiler and std.
+To use a compiler built entirely from source with the in-tree compiler and std, you need to build the
+stage2 compiler, which is compiled using the stage1 (in-tree) compiler and std.
diff --git a/src/doc/rustc-dev-guide/src/building/how-to-build-and-run.md b/src/doc/rustc-dev-guide/src/building/how-to-build-and-run.md
index c3c1c41e3f6..c4783002b85 100644
--- a/src/doc/rustc-dev-guide/src/building/how-to-build-and-run.md
+++ b/src/doc/rustc-dev-guide/src/building/how-to-build-and-run.md
@@ -217,7 +217,6 @@ probably the best "go to" command for building a local compiler:
 This may *look* like it only builds the standard library, but that is not the case.
 What this command does is the following:
 
-- Build `std` using the stage0 compiler
 - Build `rustc` using the stage0 compiler
   - This produces the stage1 compiler
 - Build `std` using the stage1 compiler
@@ -241,8 +240,7 @@ build. The **full** `rustc` build (what you get with `./x build
 --stage 2 compiler/rustc`) has quite a few more steps:
 
 - Build `rustc` with the stage1 compiler.
-  - The resulting compiler here is called the "stage2" compiler.
-- Build `std` with stage2 compiler.
+  - The resulting compiler here is called the "stage2" compiler, which uses stage1 std from the previous command.
 - Build `librustdoc` and a bunch of other things with the stage2 compiler.
 
 You almost never need to do this.
@@ -250,14 +248,14 @@ You almost never need to do this.
 ### Build specific components
 
 If you are working on the standard library, you probably don't need to build
-the compiler unless you are planning to use a recently added nightly feature.
-Instead, you can just build using the bootstrap compiler.
+every other default component. Instead, you can build a specific component by
+providing its name, like this:
 
 ```bash
-./x build --stage 0 library
+./x build --stage 1 library
 ```
 
-If you choose the `library` profile when running `x setup`, you can omit `--stage 0` (it's the
+If you choose the `library` profile when running `x setup`, you can omit `--stage 1` (it's the
 default).
 
 ## Creating a rustup toolchain
@@ -271,7 +269,6 @@ you will likely need to build at some point; for example, if you want
 to run the entire test suite).
 
 ```bash
-rustup toolchain link stage0 build/host/stage0-sysroot # beta compiler + stage0 std
 rustup toolchain link stage1 build/host/stage1
 rustup toolchain link stage2 build/host/stage2
 ```
diff --git a/src/doc/rustc-dev-guide/src/building/new-target.md b/src/doc/rustc-dev-guide/src/building/new-target.md
index 09ffbe8c882..8d323ba9646 100644
--- a/src/doc/rustc-dev-guide/src/building/new-target.md
+++ b/src/doc/rustc-dev-guide/src/building/new-target.md
@@ -85,7 +85,7 @@ Look for existing targets to use as examples.
 After adding your target to the `rustc_target` crate you may want to add
 `core`, `std`, ... with support for your new target. In that case you will
 probably need access to some `target_*` cfg. Unfortunately when building with
-stage0 (the beta compiler), you'll get an error that the target cfg is
+stage0 (a precompiled compiler), you'll get an error that the target cfg is
 unexpected because stage0 doesn't know about the new target specification and
 we pass `--check-cfg` in order to tell it to check.
 
diff --git a/src/doc/rustc-dev-guide/src/building/suggested.md b/src/doc/rustc-dev-guide/src/building/suggested.md
index f8a28b7f2e9..333554c8a90 100644
--- a/src/doc/rustc-dev-guide/src/building/suggested.md
+++ b/src/doc/rustc-dev-guide/src/building/suggested.md
@@ -310,51 +310,15 @@ lets you use `cargo fmt`.
 [the section on vscode]: suggested.md#configuring-rust-analyzer-for-rustc
 [the section on rustup]: how-to-build-and-run.md?highlight=rustup#creating-a-rustup-toolchain
 
-## Faster builds with `--keep-stage`.
-
-Sometimes just checking whether the compiler builds is not enough. A common
-example is that you need to add a `debug!` statement to inspect the value of
-some state or better understand the problem. In that case, you don't really need
-a full build. By bypassing bootstrap's cache invalidation, you can often get
-these builds to complete very fast (e.g., around 30 seconds). The only catch is
-this requires a bit of fudging and may produce compilers that don't work (but
-that is easily detected and fixed).
-
-The sequence of commands you want is as follows:
-
-- Initial build: `./x build library`
-  - As [documented previously], this will build a functional stage1 compiler as
-    part of running all stage0 commands (which include building a `std`
-    compatible with the stage1 compiler) as well as the first few steps of the
-    "stage 1 actions" up to "stage1 (sysroot stage1) builds std".
-- Subsequent builds: `./x build library --keep-stage 1`
-  - Note that we added the `--keep-stage 1` flag here
-
-[documented previously]: ./how-to-build-and-run.md#building-the-compiler
-
-As mentioned, the effect of `--keep-stage 1` is that we just _assume_ that the
-old standard library can be re-used. If you are editing the compiler, this is
-almost always true: you haven't changed the standard library, after all. But
-sometimes, it's not true: for example, if you are editing the "metadata" part of
-the compiler, which controls how the compiler encodes types and other states
-into the `rlib` files, or if you are editing things that wind up in the metadata
-(such as the definition of the MIR).
-
-**The TL;DR is that you might get weird behavior from a compile when using
-`--keep-stage 1`** -- for example, strange [ICEs](../appendix/glossary.html#ice)
-or other panics. In that case, you should simply remove the `--keep-stage 1`
-from the command and rebuild. That ought to fix the problem.
-
-You can also use `--keep-stage 1` when running tests. Something like this:
-
-- Initial test run: `./x test tests/ui`
-- Subsequent test run: `./x test tests/ui --keep-stage 1`
-
-### Iterating the standard library with `--keep-stage`
-
-If you are making changes to the standard library, you can use `./x build
---keep-stage 0 library` to iteratively rebuild the standard library without
-rebuilding the compiler.
+## Faster Builds with CI-rustc  
+
+If you are not working on the compiler, you often don't need to build the compiler tree.
+For example, you can skip building the compiler and only build the `library` tree or the
+tools under `src/tools`. To achieve that, you have to enable this by setting the `download-rustc`
+option in your configuration. This tells bootstrap to use the latest nightly compiler for `stage > 0`
+steps, meaning it will have two precompiled compilers: stage0 compiler and `download-rustc` compiler
+for `stage > 0` steps. This way, it will never need to build the in-tree compiler. As a result, your
+build time will be significantly reduced by not building the in-tree compiler.
 
 ## Using incremental compilation
 
diff --git a/src/doc/rustc-dev-guide/src/tests/ci.md b/src/doc/rustc-dev-guide/src/tests/ci.md
index 825be11c82a..8f259f27044 100644
--- a/src/doc/rustc-dev-guide/src/tests/ci.md
+++ b/src/doc/rustc-dev-guide/src/tests/ci.md
@@ -66,9 +66,9 @@ kinds of builds (sets of jobs).
 ### Pull Request builds
 
 After each push to a pull request, a set of `pr` jobs are executed. Currently,
-these execute the `x86_64-gnu-llvm-X`, `x86_64-gnu-tools`, `mingw-check` and
-`mingw-check-tidy` jobs, all running on Linux. These execute a relatively short
-(~30 minutes) and lightweight test suite that should catch common issues. More
+these execute the `x86_64-gnu-llvm-X`, `x86_64-gnu-tools`, `mingw-check-1`, `mingw-check-2`
+and `mingw-check-tidy` jobs, all running on Linux. These execute a relatively short
+(~40 minutes) and lightweight test suite that should catch common issues. More
 specifically, they run a set of lints, they try to perform a cross-compile check
 build to Windows mingw (without producing any artifacts) and they test the
 compiler using a *system* version of LLVM. Unfortunately, it would take too many